US Republican presidential candidate Donald Trump has promised to foil a proposed deal for AT&T to buy Time Warner if he wins the Nov 8 election, arguing it was an example of a "power structure" rigged against both him and voters.

Trump, whose candidacy has caused ruptures in his party, listed his policy plans on Saturday for the first 100 days of his presidency in a campaign speech in Gettysburg, Pennsylvania, near the site of a Civil War battlefield and a celebrated address by President Abraham Lincoln.

But he also defiantly raised personal grievances, describing how, if elected, he would address them from the White House in a way he said would benefit Americans.

Moments after promising Americans that he represented a hopeful break from the status quo, he promised to sue nearly a dozen women who have come forward in the last two weeks to accuse him of sexual assault, calling them liars.

And he added a new threat to his repeated castigation of US media corporations, which he says cover his campaign unfairly to help Democratic candidate Hillary Clinton.

"They're trying desperately to suppress my vote and the voice of the American people," Trump, who often rails against media outlets and journalists covering his events, told supporters in his speech. Trump has not provided evidence for his assertion that the election would be rigged.

"As an example of the power structure I'm fighting, AT&T is buying Time Warner and thus CNN, a deal we will not approve in my administration because it's too much concentration of power in the hands of too few," Trump said.

Telecommunications company AT&T Inc has agreed in principle to buy Time Warner Inc, one of the country's largest film and television companies, for about $85 billion and an announcement could be made as early as Saturday.

Trump also said he would look at "breaking" up the acquisition by Comcast Corp of the media company NBC Universal in 2013.

"Deals like this destroy democracy," he said in explaining his apparent deviation from the traditional Republican position that seeks to minimize the taxation and regulation of American companies.

Trump, a wealthy New York building developer and television star, acknowledged in a debate with Clinton on Oct 9 that he had used investment losses to avoid paying taxes. The New York Times reported on Oct 1 that Trump's declared loss of $916 million in 1995 was so large that he could legally have avoided paying any federal income taxes for up to 18 years.

Although Trump on Saturday described his plans at least in part as a response to his belief such organizations had treated him unfairly, he argued that less wealthy voters had even greater cause to worry.

"When a simple phone call placed with the biggest newspapers or television networks gets them wall-to-wall coverage with virtually no fact-checking whatsoever, here is why this is relevant to you," he said. "If they can fight somebody like me who has unlimited resources to fight back, just look at what they could do to you, your jobs, your security, your education, your health care."

Trump, who has said he may not accept the election's outcome if he loses, is trailing Clinton in most polls - although he has narrowed the gap according to a Reuters/Ipsos poll released on Friday. Clinton maintained her commanding lead in the race to win the Electoral College, however, and claim the US presidency, a Reuters/Ipsos States of the Nation project poll released on Saturday showed.

The speech was billed by his campaign as major outlining of his policies and principles. Many of the policy ideas Trump listed on Saturday were familiar, not least his promise to build a wall on the border with Mexico to deter illegal immigration.

Trump has bluntly said that Mexico will pay for the wall, an idea the Mexican government has scoffed at. He tweaked his language on Saturday, saying the United States would fully fund the wall with the understanding that Mexico would reimburse the cost.

He also proposed a new mandatory minimum sentence of two years in federal prison for anyone caught re-entering the country after deportation. Trump reiterated his promises to use tariffs to discourage American companies from shifting operations abroad, to repeal the Affordable Care Act and to increase spending to improve infrastructure.

The Clinton campaign mocked the speech, saying his only new policy was to promise to sue his female accusers.

"Like Trump's campaign, this speech gave us a troubling view as to what a Trump State of the Union would sound like," spokeswoman Christina Reynolds said in a statement. She called the speech "rambling, unfocused, full of conspiracy theories and attacks on the media, and lacking in any real answers for American families."

Trump's campaign was thrown into crisis two weeks ago when a 2005 video was released showing him bragging about groping and kissing women, prompting several prominent Republicans to announce they would not vote for him.

Since then, at least 10 women have said Trump made unwanted sexual advances, including groping or kissing, in incidents from the early 1980s to 2007, all of which Trump has denied.

Trump, 70, said he was being attacked because he was an outsider who had never previously run for office, which he argues is a virtue.

"The fact that Washington and the Washington establishment has tried so hard to stop our campaign is only more proof that our campaign represents the kind of change that only arrives once in a lifetime," he said.